8.4 paging Paging is a memory-management

scheme that permits the physical address space of a process to be non-contiguous.

The basic method for implementation involves breaking physical memory into fixed-sized blocks called FRAMES and break logical memory into blocks of the same size called PAGES

Paging

Every address generated by the CPU is divided into two parts: Page number (p) and Page offset (d)

The page number is used as an index into a Page Table

Paging

Paging

Paging The page size is defined by the hardware The size of a page is typically a power of

2, varying between 512 bytes and 16MB per page

Reason: If the size of logical address is 2^m and page size is 2^n, then the high-order

m-n bits of a logical address designate the page number

Paging

Paging Example

Paging When we use a paging scheme, we

have no external fragmentation: ANY free frame can be allocated to a process that needs it.

However, we may have internal fragmentation

For example: if a page size is 2048 bytes, a process of 72766 bytes would need 35 pages plus 1086 bytes

Paging

If the process requires n pages, at least n frames are required

The first page of the process is loaded into the first frame listed on free-frame list, and the frame number is put into page table

Paging

Hardware Support on Paging To implement paging, the simplest

method is to implement the page table as a set of registers

However, the size of register is limited and the size of page table is usually large

Therefore, the page table is kept in main memory

Hardware Support on Paging If we want to access location I, we must

first index into page table, this requires one memory access

With this scheme, TWO memory access are needed to access a byte

The standard solution is to use a special, small, fast cache, called Translation look-aside buffer (TLB) or associative memory

TLB

TLB If the page number is not in the TLB

(TLB miss) a memory reference to the page table must be made.

In addition, we add the page number and frame number into TLB

If the TLB already full, the OS have to must select one for replacement

Some TLBs allow entries to be wire down, meaning that they cannot be removed from the TLB, for example kernel codes

TLB The percentage of times that a particular page

number is found in the TLN is called hit ratio If it takes 20 nanosecond to search the TLB

and 100 nanosecond to access memory

If our hit ratio is 80%, the effective memory access time equal:

0.8*(100+20) + 0.2 *(100+100)=140 If our hit ratio is 98%, the effective memory

access time equal: 0.98*(100+20) + 0.02 *(100+100)=122 (detail in CH9)

Memory Protection

Memory protection implemented by associating protection bit with each frame

Valid-invalid bit attached to each entry in the page table:

“valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page

“invalid” indicates that the page is not in the process ’logical address space

Memory Protection

Suppose a system with a 14bit address space (0 to 16383), we have a program that should use only address 0 to 10468. Given a page size of 2KB, we may have the following figure:

Memory Protection

Memory Protection Any attempt to generate an

address in page 6 or 7 will be invalid

Notice that this scheme allows the program to access 10468 to 12287, this problem is result of the 2KB page size and reflects the internal fragmentation of paging

Shared Pages An advantage of paging is the possible

of sharing common code, especially time-sharing environment

For example a server with 40 user using text editor (with 150k reentrant code and 50k data space)

In next figure, we see three page editor with 50k each. Each process has its own data page

Shared Pages

Shared Pages

In this case, we need only 150k + 40* 50k = 2150 KB

Instead of (150k + 50K)*40 = 8000KB

8.5 Structure of Page Table Consider a system with 32-bit

logical address space. If the page size is 4 KB (2^12),

then the page table may consist of up to 1 million entries (2^32/2^12=2^20)

Clearly, we would not want to allocate the page table contiguously in main memory

Hierarchical paging

One way is to use a two-level paging algorithm

Hierarchical paging Remember the example is a 32-bit

machine with a page size of 4 KB. A logical address is divided into a

page number consisting of 20 bits and a page offset consisting of 12 bits

10 10 12

Hierarchical paging

Address translation scheme:

Hierarchical paging

For a system with 64-bit system, by the same scheme with page size=4KB will looks like:

Hierarchical paging

One way to avoid such large table is to further divide the outer table into smaller pieces:

Hierarchical Paging

The next step would be a four-level paging scheme…

However, for 64-bit architecture, hierarchical page table are generally considered inappropriate

Hash Page Table

Hash Page table is commonly used in systems with address spaces larger than 32 bit

So what is a hash table?

Hash Page Table

Each element consists of three fields:

1. The virtual page number2. The value of the mapped page

frame3. A pointer to the next element

Hash Page Table

8.6 Segmentation Since the user’s view of memory is not

the same as the actual physical memory, segmentation helps user to view memory as a collection of variable-size segment

Segmentation is a memory management scheme that supports user view of memory

Segmentation

A program is a collection of segments. A segment is a logical unit such as:

main program, procedure, function, method, object, local variables, global variables, common block, stack, symbol table, arrays

Segmentation

Segmentation

Segmentation The user specifies each address by two

quantities: a segment name and an offset

<segment-number, offset>

Compare with page scheme, user specifies only a single address, which is partitioned by hardware into a page number and an offset, all invisible to the programmer

Segmentation Although the user can refer to objects in

the program by a two-dimensional address, the actual physical address is still a one-dimensional sequence

Thus, we need to map the segment number

This mapping is effected by a segment table

In order to protect the memory space, each entry in segment table has a segment base and a segment limit

Segmentation Hardware

Example of Segmentation For example, segment 2

starts from 4300 with size 400, if we reference to byte 53 of segment 2, it mapped to 4335

A reference to segment 3, byte 852?

A reference to segment 0, byte 1222?

8.7 Example: Intel Pentium In Pentium system, the CPU generates

logical address, which are given to the segmentation unit

The segmentation unit produces a linear address for each logical address

Then the linear address is then given to paging unit, which in turn generates the physical address in main memory.

Pentium paging

The Pentium architecture allows a page size of either 4KB or 4MB.

For 4KB pages, the Pentium uses a two level paging scheme in which division of the 32 bit linear address as:

Pentium paging

Linux on Pentium System Although the Pentium uses a two-level

paging model, Linux is designed to run on a variety of hardware platform, many of which are 64-bit platforms

Therefore, two-level paging is not plausible

Thus, Linux has adopted a three level paging strategy that works well for both 32-bit and 64-bit architectures

Linux on Pentium System The linear address is Linux is broken

into the following four parts:

The number of bits in each part varies according to architecture

How does Linux apply its three-level model on the Pentium? The size of middle directory is zero bits